High reliability pressure sensor

ABSTRACT

The reliability of a pressure sensor is improved either by utilization of redundant composants. A pair of pressure sensors are mounted upon a single pressure sensor diaphragm. The pressure signals generated by the pressure sensors are compared and, if the difference between the signals exceeds a predetermined threshold, it is determined that a malfunction of the pressure sensor has ocurred. Alternately, additional diagnostic testing may be included to detect a malfunctioning sensor.

BACKGROUND OF THE INVENTION

[0001] This invention relates in general to pressure sensors used inhydraulic control systems and in particular to a high reliabilitypressure sensor utilized in a vehicle brake system with Hydraulic BrakeAssist.

[0002] Recently, Hydraulic Brake Assist (HBA) has been included on newvehicles. HBA provides maximum braking capability during an emergencybraking situation. During a braking cycle, the brake pressure is sensedto determine if an emergency situation has occurred. Alternately, themagnitude of the brake pedal stroke and speed of brake pedal movementcan be monitored for an emergency braking situation. Typically, anemergency is identified by a certain pedal-application speed occurringalong with a minimum level of brake-pedal force. Thus, a quick, deepstab at the brake pedal actives HBA while a quick shallow stab, as tocancel cruise control, or a slow but deep pedal application, as whenslowing for a curve, will not active HBA.

[0003] Upon detection of an emergency braking situation, HBA increasesbrake application pressure to a maximum value and continues to hold themaximum pressure until the vehicle stops or the brake pedal is released,as illustrated in FIG. 1. In FIG. 1, vehicle braking force is plotted asa function of time. The lower curve, which is labeled 4, is for a brakesystem without HBA, while the upper curve, which is labeled 6, is for abrake system that includes HBA. Typically, during an emergency brakingsituation, the vehicle operator partially lifts his foot from the brakepedal following his initial quick, deep stab. Thus, HBA assures that thebrakes remain applied with maximum force.

[0004] There are a number of know methods for integrating HBA with avehicle brake system. One method is completely mechanical and involvesmodification of the vacuum brake booster to provide HBA. Another methodis to include the HBA function in an Anti-lock Brake System (ABS). AnABS is often included in many vehicles to prevent wheel lock up duringstops upon low mu road surfaces. Such systems detect excessive slippageof one or more controlled wheels and selectively reduce and reapply thepressure applied to the controlled wheel brakes to reduce the slippageand thereby avoid a potential locking-up of the wheel.

[0005] Referring again to the drawings, there is illustrated in FIG. 2,a typical brake control system 10 which has HBA included in an Anti-lockBrake System (ABS). The brake control system 10 is intended to beexemplary and it will be appreciated that there are other brake controlsystems having different architecture than shown. In FIG. 2, a brakepedal 12 is mechanically coupled (not shown) to a brake light switch 13and a dual reservoir master cylinder 14. The master cylinder 14 isconnected to a hydraulic control unit 16 by a pair of hydraulic lines 18and 20. The hydraulic control unit 16 includes a plurality of solenoidvalves to control the brake pressure applied to the individual wheelbrakes. The control unit 16 also typically includes a source ofpressurized hydraulic fluid, such as a pump driven by an electric motor.The control unit 16 is connected via hydraulic lines 22, 24, 26 and 27to individual wheel brakes (not shown) for the front wheels 28 and 30and the rear wheels 32 and 33. Typically, the brake circuit isdiagonally split with one master cylinder reservoir controlling thebrakes associated with the left front wheel 30 and right rear wheel 33and the other master cylinder reservoir controlling the brakesassociated with the right front wheel 28 and the left rear wheel 32.

[0006] The brake control system 10 also includes a pair of front wheelspeed sensors 34 that generate signals that are proportional to thespeed of the front wheels 28 and 30 and a pair of rear wheel speedsensors 36 that generate signals that are proportional to the speed ofthe rear wheels 32 and 33. The wheel speed sensors 34 and 36 and thestop light switch 13 are electrically connected to an Electronic ControlUnit (ECU) 38.

[0007] The control unit 38 includes a microprocessor (not shown), that,under the control of an algorithm, selectively actuates the solenoidvalves and pump in the control unit 16 to correct excessive wheelslippage.

[0008] The brake control system 10 further includes a pressure sensor 40that monitors the hydraulic pressure in one of the master cylinderreservoirs. An pressure signal is supplied to the ECU 38. Themicroprocessor monitors the pressure signal and responsive thereto, upondetecting an emergency brake application, to actuate HBA.

[0009] A typical prior art pressure sensor assembly is illustratedgenerally at 44 in FIG. 3. The pressure sensor assembly includes asensor element 46 that is electrically coupled to an ApplicationSpecific Integrated Circuit (ASIC) 47. Hydraulic pressure is applied tothe sensor element 46. Both the sensor element 46 and the ASIC 47 aretypically mounted in a common housing, that is shown schematically bythe dashed line labeled 48 in FIG. 3. The sensor element 46 may includea plurality of strain gauges mounted upon one side of a thin diaphragm.The diaphragm is usually a disc formed from stainless steel. The straingauges are typically arranged as a conventional half or full bridgecircuit, such as, for example, a conventional thin film WheatstoneBridge. The hydraulic brake fluid in the brake system is in contact withthe side of the diaphragm opposite from the strain gauges. When thevehicle brakes are applied, the hydraulic brake fluid is pressurized andcauses the diaphragm to deflect from its rest position. As the diaphragmis deflected by the applied pressure, the strain gauges are stretched orcompressed, causing a change in the internal resistance of the gauges.The changed resistances result in a voltage appearing across the bridgecircuit that is proportional to the magnitude of the pressure. Thevoltage is conditioned by the ASIC 47. The ASIC 47 generates an analogor digital pressure signal that is applied to an input port of an ECUmicroprocessor 49. The microprocessor 49 is included in the vehiclebrake control system 10.

SUMMARY OF THE INVENTION

[0010] This invention relates to a high reliability pressure sensorutilized in a vehicle brake system with Hydraulic Brake Assist.

[0011] As explained above, current HBA systems include a pressure sensorto detect an emergency stop condition. However, if the pressure sensorshould malfunction or fail, it is possible that a false emergency stopsignal may be generated that would trigger the HBA. It is known toimprove HBA system reliability by including a second complete pressuresensor to provide a redundant pressure signal to the ECU microprocessor.The ECU microprocessor compares the two signals and, if the signals aredifferent, it is assumed that one of the pressure sensors ismalfunctioning and the HBA is disabled. However, the inclusion of twocomplete pressure sensors is both bulky and expensive. Two pressuresensors also require two ports in the hydraulic control unit whichincreases the potential for hydraulic fluid leakage. Accordingly, itwould be desirable to improve the reliability of the measurement of thebrake pressure in a HBA system without requiring two separate pressuresensors.

[0012] The present invention contemplates a pressure sensor assembly fora hydraulic control unit that includes a pressure sensor housing adaptedto be mounted upon a hydraulic control unit and a pressure sensordiaphragm carried by the housing. First and second pressure sensingelements are mounted upon the pressure sensor diaphragm. A first signalconditioning circuit is connected to the first pressure sensing elementand a second signal conditioning circuit is connected to the secondpressure sensing element The said first and second signal conditioningcircuits are operable to generate first and second pressure signals atoutput ports. An active electronic device is connected to the outputports of the first and second signal conditioning circuits and isoperative to compare the first and second pressure signals. In thepreferred embodiment, the active electronic device includes amicroprocessor; however, other devices, such as, for example, acomparator circuit also can be used. Upon detecting a difference betweenthe pressure signals the electronic device generates an error signal.The error signal can be generated when the difference between thepressure signals is non-zero or when the difference exceeds apredetermined threshold In the preferred embodiment, the pressure sensorassembly is included in a hydraulic brake assist system and theelectronic device is further operable to disable the hydraulic brakeassist system upon generating the error signal. The first and secondsignal conditioning circuits can be separate electronic components orcan be included in a single electronic component.

[0013] Alternately, the two pressure sensing elements can be connectedto a single signal conditioning circuit. The signal conditioning circuitis operable to generate a digital pressure signal which includespressure data from both of the pressure sensing elements. In thepreferred embodiment, the digital pressure signal is time multiplexed.Additionally, the pressure sensor assembly can include a temperaturesensor with the digital signal generated by the signal conditioningcircuit including temperature data.

[0014] It is further contemplated that the pressure sensor assemblyincludes a single pressure sensing element connected to a signalconditioning circuit. The signal conditioning circuit being operative togenerate a pressure signal. The signal conditioning circuit alsoincludes at least one diagnostic test and is operable to generate anerror signal upon detecting a predetermined fault condition.Furthermore, the associated active electronic device also can include atleast one diagnostic test and be operative to generate an error signalupon detection of a predetermined fault condition. Additionally, theactive electronic device can be adapted to receive operating data fromat least one vehicle component and to include the vehicle parameter datain the diagnostic test.

[0015] The present invention also contemplates a method for measuring apressure that includes providing a pressure sensor having first andsecond sensor elements. The first and second sensor elements areconnected to corresponding first and second signal conditioning circuitswith the second signal conditioning circuit having a gain that is afraction of the gain of the first signal conditioning circuit. Theoutput of the first signal conditioning circuit is compared to a firstthreshold. Upon the output of the first signal conditioning circuitexceeding the first threshold, the output of the second signalconditioning circuit is compared to a second threshold, and if theoutput of the second signal conditioning circuit exceeds the secondthreshold, an error flag is set.

[0016] Various objects and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the preferred embodiment, when read in light of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a plot of braking force vs. time that illustrates theoperation of Hydraulic Brake Assist.

[0018]FIG. 2 is a schematic diagram of a brake control system inaccordance with the prior art that includes Hydraulic Brake Assist.

[0019]FIG. 3 is a schematic diagram of a pressure sensor that isincluded in the brake control system shown in FIG. 2.

[0020]FIG. 4 is a schematic diagram of a pressure sensor for the brakecontrol system shown in FIG. 2 that is in accordance with the presentinvention.

[0021]FIG. 5 is a sectional view of the pressure sensor shown in FIG. 4.

[0022]FIG. 6 is a schematic diagram of an alternate embodiment of thepressure sensor shown in FIG. 2.

[0023]FIG. 7 is a schematic diagram of another alternate embodiment ofthe pressure sensor shown in FIG. 2.

[0024]FIG. 8 is a schematic diagram of another alternate embodiment ofthe pressure sensor shown in FIG. 2.

[0025]FIG. 9 is a flow chart for an algorithm for the operation of thepressure sensor illustrated in FIG. 8.

[0026]FIG. 10 is a flow chart for a method for failure detection of thepressure sensor shown in FIG. 4.

[0027]FIG. 11 is a graph of voltage vs. pressure that illustrates amethod for failure detection of the pressure sensor shown in FIG. 4.

[0028]FIG. 12 is a graph of percent of saturation voltage vs. pressurethat illustrates a method for failure detection of the pressure sensorshown in FIG. 4.

[0029]FIG. 13 is a flow chart for an alternate embodiment of the methodfor failure detection shown in FIG. 10.

[0030]FIG. 14 is a flow chart for another alternate embodiment of themethod for failure detection shown in FIG. 10.

[0031]FIG. 15 is a flow chart for an alternate method for failuredetection of the pressure sensor shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] Referring once more to the drawings, there is shown at 50 in FIG.4, a schematic diagram for an improved pressure sensor that increasesHBA system reliability by providing redundancy in a single pressuresensor assembly. As best seen in FIG. 5, the present inventioncontemplates placing two separate conventional sensor elements 52 and 54upon a single thin diaphragm 56 within a single pressure sensor housing58. It will be appreciated that the pressure sensor structure shown inFIG. 5 is meant to be exemplary. The pressure sensor also can beconfigured differently than shown in FIG. 5, such as, for example,pressure sensor elements can be glued to a surface of a cavity that isthen filled with a transfer fluid (not shown). A thin diaphragm wouldseparate the transfer fluid from the brake fluid. Any change in brakefluid pressure would be trnsmitted through the diaphragm to the transferfluid. Changes in transfer fluid pressure would be detected by thepressure sensor elements. However, two sets of pressure sensor elementswould be included in the cavity. As also shown in FIG. 5, the pressuresensing housing 58 is mounted upon a Hydraulic Control Unit (HCU) 16;however, it will be appreciated that the pressure sensor also can beintegrally mounted within the HCU 16 not shown). Each of the sensorelements 52 and 54 generates a bridge voltage that is applied to aninput port of an associated sensor ASIC 60 and 62 mounted upon a PrintedCircuit Board (PCB) 64.

[0033] As shown in FIG. 4, the first ASIC 60 conditions the bridgevoltage generated by the associated sensor element 52 to obtain a firstanalog pressure output signal while the second ASIC 62 conditions thebridge voltage generated by the associated sensor element 62 to obtain asecond analog pressure output signal. The first conditioned outputpressure signal is supplied through an electrical connector 66 and overa first analog signal line 68 to a corresponding pressure input port 70of an ECU microprocessor 72. Similarly, the second conditioned outputpressure signal is supplied through the electrical connector 66 and overa second analog signal line 74. to a corresponding pressure input port76 of the ECU microprocessor 72. Alternately, the ASIC's 60 and 62 cangenerate digital pressure output signals, in which case a digital signalline would connect the each of the ASIC's to the microprocessor 72.Additionally, in the preferred embodiment, the connector 66 includescontacts for power supply and ground connections (not shown).

[0034] The ECU microprocessor 72 compares the two output pressuresignals, and, if the signals are different, determines that the sensorassembly 50 has malfunctioned. Upon determination that the sensorassembly 50 has malfunctioned, the microprocessor 72 disables the HBAand generates a warning signal for the vehicle operator. In thepreferred embodiment, the warning signal consists of illuminating alight on the vehicle dashboard (not shown). The invention contemplatesusing one of two modes of operation for the microprocessor testcomparison. In the first mode, the microprocessor 72 determines that amalfunction has occurred if the two pressure signals are not identical,that is, the difference between the two pressure signals is non-zero.Alternately, the microprocessor 72 can determine that a malfunction hasoccurred if the difference between the two pressure signals is greaterthan a predetermined threshold As long as the difference between thepressure signals is within the allowable range, the microprocessor 72accepts the pressure signal data as correct and generates a digitalestimated pressure signal.

[0035] In the preferred embodiment, the microprocessor 72 is continuallymonitoring the two pressure signals while the vehicle is being operated.Thus, the pressure sensor 50 shown in FIGS. 4 and 5 provides physicalredundancy by including two sensor elements, 52 and 54, two ASIC's, 60and 62, and two analog signal transmission lines, 68 and 74.

[0036] The invention also contemplates an alternate structure forproviding increased HBA system reliability with a single pressure sensorassembly for a HBA, as shown generally at 78 in FIG. 6. Components inFIG. 6 that are similar to components shown in FIG. 4 have the samenumerical designators. Similar to the previous pressure sensor 50, thealternate structure 78 includes two pressure sensing elements 52 and 54mounted upon a single pressure sensor diaphragm. Each of the sensorelements 52 and 54 generates a bridge voltage that is applied to acorresponding input port of a single chip 79 mounted upon the PrintedCircuit Board (PCB) 64 (not shown). The chip 79 is formed to include twoseparate ASIC portions, that are labeled ASCI #1 and ASCI #2 in FIG. 6.Each of the ASIC portions conditions the bridge voltage of one of thesensor elements 52 and 54 to obtain two analog pressure output signals.As described above, the two conditioned output pressure signals aresupplied through an electrical connector 66 over two analog lines 68 and74 to two corresponding pressure input ports 70 and 76 of the ECUmicroprocessor 72. By combining the two ASIC's into a single chip, thenumber of components is reduced while the redundancy of the sensorelements 42 and 54 and transmission lines 68 and 74 is maintained. Asdescribed above, the microprocessor 72 compares the two analog pressuresignals to determine if the pressure sensor is functioning properly.

[0037] The invention also contemplates a second alternate structure forproviding increased HBA system reliability with a single pressure sensorassembly for a HBA, as shown generally at 80 in FIG. 7. Components inFIG. 7 that are similar to components shown in FIG. 4 have the samenumerical designators. Similar to the previously described system 50,the alternate system 80 includes two pressure sensing elements 52 and 54mounted upon a single pressure sensor diaphragm However, the bridgevoltages generated by the two sensing elements 52 and 54 are applied toinput ports of a single signal conditioning ASIC 82. The ASIC 82digitizes and conditions the bridge voltages. The digitized pressuresignals are combined into a time-multiplexed signal and transmitted overa single transmission line 84 to a single pressure input port 85 of anECU microprocessor 86.

[0038] The microprocessor 86 compares the pressure signals and if thedifference between the signals is greater than a predeterminedthreshold, the microprocessor 86 generates an error signal and disablesthe HBA. Upon the HBA being disabled, a HBA failure indicator isilluminated to warn the vehicle operator. In the preferred embodiment,the pressure signals are transmitted every 3 milliseconds; however,other transmission time periods may be used. Additionally, the inventioncontemplates that the brake fluid temperature is also sensed andtransmitted to the microprocessor 86; however, the temperature sensingis optional. The use of a time-multiplexed signal allows a two-wireconnection between the sensor and the microprocessor 86 with currentswitching similar to an active wheel sensor. In the preferredembodiment, the pressure sensor 80 is compatible with a 100K bauduniversal asynchronous receiver/transmitter line. Furthermore, as anoption, the ASCI 82 can be programmed to periodically sendmanufacturer's calibration data and serial number to the ECU. This iscontemplated as being done less frequently than the pressure andtemperature data transmittal, such as, for example at one secondintervals.

[0039] The invention further contemplates a third alternate structurefor providing increased HBA system reliability in a single pressuresensor assembly for a HBA, as shown generally at 90 in FIG. 8. Thepressure sensor 90 has a single pressure sensor element 92 and a singlesensor signal conditioning ASIC 94. The ASIC 94 generates an analogpressure signal that is applied to a single pressure input port 96 of anECU microprocessor 98. The ASIC 94 is programmed to apply diagnostictests itself and to the bridge voltage generated by the sensor 90. Upondetecting an improper operating condition, the sensor ASIC 94 willgenerate an error signal to cause the ECU microprocessor 98 to disablethe HBA. Additionally, the ECU microprocessor 98 continuously appliesdiagnostic tests to the pressure signal received from the sensor ASIC94. Upon detecting a signal which is outside of an allowable operatingrange, the microprocessor 98 generates an error signal and disables theHBA. Upon disabling the HBA, the microprocessor 98 also illuminates awarning light to inform the vehicle operator of the problem.

[0040] A typical diagnostic tests will be described next. In thepreferred embodiment, a regulated voltage supply supplies power to thepressure sensor and the bridge circuit output voltage is within therange that is greater than zero but less than the supply voltage. Thus,one of the diagnostic tests can include continuously monitoring thebridge output voltage to determine if the bridge output voltage isoutside of the expected voltage range. For example, a determination thatthe bridge voltage is zero is an indication of a possible short circuitin the bridge while a voltage that is equal to the supply voltage is anindication of a possible open circuit in the bridge circuit. Uponencountering one of these conditions, the ASIC 94 would generate anerror signal. The microprocessor 98 would be responsive to the errorsignal to disable the HBA.

[0041] The invention also contemplates that the diagnostic tests coulddetermine in-range failures, that is failures that could occur with thebridge output voltage remaining within the allowable bridge outputvoltage range. Thus, if the diagnostics detect an output voltage readingindicating an increased pressure that has an unusually long duration,the diagnostic tests determine that the sensor assembly 90 has failedand the HBA is disabled. Accordingly, the ASIC 94 would generate anerror signal.

[0042] The present invention also contemplates that diagnostic tests canbe included in the ECU microprocessor 98. It is further contemplatedthat the microprocessor 98 can set different fault flags to aid atechnician in determining the specific cause of the fault. The testsalso can correlate with external conditions, such as, for example,whether or not the vehicle brakes are applied. Thus, an increasedpressure reading that occurs without the vehicle brakes being applied isan indication that the sensor 90 has probably failed.

[0043] It will be appreciated that the above described diagnostic testsare intended to be exemplary and that the invention also can bepracticed with other specific diagnostic tests. The replacement of twocomplete pressure sensors with one allows a corresponding reduction ofthe overall size of the hydraulic control unit. Furthermore, with thecontinuing miniaturization and reduction of costs for the associatedASCI's, it is expected that the present invention will also result inreduced costs manufacturing costs for the HBA system. The inventorsbelieve that sufficient tests can be developed to assure that the levelof reliability required for single sensor 90 to be utilized in a HBA canbe achieved.

[0044] A flow chart for a testing algorithm that includes N diagnostictests is illustrated in FIG. 9. It is contemplated that the algorithmwould be included as a subroutine in the ABS control algorithm. Thealgorithm is called periodically by the main control algorithm andentered through block 100. An index I is initialized as one infunctional block 102. In functional block 104, diagnostic test (I) isperformed. For example, the current output voltage of the sensor bridgecircuit is measured. The results of diagnostic test (I) are compared tothe test criteria in decision block 104. For example, is the bridgeoutput voltage equal to zero? If the test criteria is not met, that is,there is a FALSE finding, the subroutine transfers to functional block106 where a disable flag is set and then exits back to the main controlalgorithm through block 107. If the test criteria is met in decisionblock 105, that is, there is a TRUE finding, the subroutine transfers tofunctional block 108 where the value of I is indexed by one. The newvalue of I is compared to the total number of diagnostic tests, N, indecision block 109. If I is less than or equal to N, all N tests havenot been performed and the subroutine returns to functional block 103 toapply the next diagnostic test, such as, for example, is the bridgeoutput voltage equal to the regulated supply voltage. If I is greaterthan N in decision block 109, all tests have been run and the subroutineexits back to the main control algorithm through block 107. As describedabove, the test criteria can include parameters determined from othervehicle components, such as, for example, are the vehicle brakesapplied? Thus, an alternate embodiment of the algorithm shown in FIG. 9would include sampling the other vehicle parameters (not shown).

[0045] For an analog pressure sensor using inexpensive availableelectronics for signal processing, both overpressure and sensor failuremay cause the output of the signal processing electronics for thepressure sensor to go to a saturation voltage, V_(cc). Sinceoverpressure conditions are to be expected, it is possible that thesoftware would misinterpret the saturation voltage as a sensor failureand latch an error code. Accordingly, the invention further contemplatesa technique for determining whether the pressure sensor is experiencinga temporary overpressure condition or has actually failed.

[0046] The present invention contemplates using different gains for thesignal processing electronics associated for each of the sensorelements. Thus, for the sensor configuration illustrated in FIG. 4, thesecond sensor ASIC 62 would have a gain that is significantly less thanthe gain for the first sensor ASIC 60. For illustrative purposes, thegain of the second ASIC 62 is assumed to be one third of the gain of thefirst ASIC 60; however, it will be appreciated that the invention alsocan be practiced with other values. Thus, during an overpressurecondition, the second ASIC 62 would show the true value of the pressurewhile the first ASIC 60 would saturate. The difference in the outputreadings would be interpreted by the software as a overpressurecondition and the software would not latch an error code. Likewise, ifboth outputs saturate, the readings would be interpreted by the softwareas a sensor failure and the software would latch an error code.

[0047] The technique would be implemented by a subroutine that isillustrated by the flow chart shown in FIG. 10. The subroutine isentered through block 110. The pressures being sensed by the sensorelements 52 and 54 are measured in functional block 112. In decisionblock 116, the output of the first ASIC 60 that is associated with thefirst sensor element 52, and indicated by P₁, is compared to thesaturation voltage V_(cc). If the output P₁ is less than the saturationvoltage V_(cc), the subroutine returns to the main algorithm through theexit block 118. If the output P₁ is equal to the saturation voltageV_(cc), the subroutine continues to decision block 120 where the outputof the second ASIC 62 that is associated with the second sensor element54, and indicated by P₂, is compared to the saturation voltage V_(cc).If the output P₂ is less than the saturation voltage V_(cc), thesubroutine returns to the main algorithm through the exit block 118. Ifthe output P₂ is equal to the saturation voltage V_(cc), the subroutinecontinues to functional block 122 where a fault flag is set. Thesubroutine then exits back to the main algorithm through the exit block118.

[0048] The invention also contemplates an alternate embodiment thatutilizes a first threshold voltage, T₁, that is established as a maximumlimit for the output of the second ASIC 62, as illustrated in FIG. 11.In FIG. 11, the saturation voltage V_(cc) is shown as beingapproximately five volts and the first threshold T₁ for the second ASIC62 is shown as being slightly more than three volts. The first faultthreshold T₁ is also illustrated in FIG. 12 as a percentage of V_(cc).Additionally, if the sensor is operating properly, a minimum voltagewill always be present at the output of the signal conditioningelectronics. Below the minimum voltage exists a second fault thresholdT₂ that is shown in FIGS. 11 and 12. The second threshold T₂ would be afraction of a volt and would be determined by the specific componentsused in the signal processing electronics. Thus, when the pressuresensor is operating properly, the output of the first ASIC 60 would haveto be between T₂ and V_(cc) and the output of the second ASIC 62 wouldhave to remain between T₁ and T₂ at all times, including when the outputof the first ASIC 60 saturates. Should the output of the first ASIC 60saturate, and the output of the second ASIC 62 is not between the firstand second fault thresholds, T₁ and T₂, the software will latch an errorfault. Similarly, if the output of first ASIC 60 falls below the secondthreshold T₂, the sensor is faulty and the software will latch an errorfault.

[0049] The alternate embodiment of the technique would be implemented bya subroutine that is illustrated by the flow chart shown in FIG. 13.Blocks in FIG. 13 that are the same as blocks shown in FIG. 10 have thesame numerical identifier. The subroutine proceeds through functionalblock 112 to the first decision block 126 where P₁ is compared to thesecond threshold, T₂. If P₁ is less than, or equal to, T₂, thesubroutine transfers to functional block 122 and the fault flag is set.If P₁ is greater than T₂, the subroutine transfers to decision block 130where P₂ is compared to the second threshold T₂. If P₂ is less than, orequal to, the second threshold T₂, the subroutine transfers tofunctional block 122 where the fault flag is set and then exits throughblock 118. If P₂ is greater than the second threshold T₂, the subroutinetransfers to decision block 116 where P₁ is compared to V_(cc). If theoutput P₁ is less than the saturation voltage V_(cc) , the subroutinereturns to the main algorithm through the exit block 118. Thus, thefirst three decision blocks, 126, 130 and 132, provide absolute testsfor the pressures P₁ and P₂. If the output P₁ is equal to the saturationvoltage V_(cc), the subroutine continues to decision block 132 where P₂is compared to the first fault threshold T₂. If P₂ is greater than, orequal to the first threshold T₁, the subroutine transfers to functionalblock 122 where the fault flag is set and then exits through block 118.If P₂ is greater than, or equal to, the second threshold T₂, the programexits through block 118.

[0050] The invention further contemplates another embodiment thatincludes, in addition to the above, comparing the outputs of the twosensor ASIC's to an error value, E. The alternate embodiment isillustrated by the flow chart shown in FIG. 14 where the blocks that arethe same as the blocks in FIGS. 10 and 13 have the same numericaldesignators. The algorithm proceeds as above until it reaches the fourthdecision block 132. In decision block 132, if P₂ is less thin T₂, thealgorithm transfers to a fifth decision block 134 where the absolutevalue of the difference between P₁ and K*P₂ is compared to the errorvalue E. It will be noted that P₁ and P₂ are voltage levels at theoutputs of the pressure sensors. In the preferred embodiment, theconstant K is the reciprocal of the fractional multiplier of the gain ofthe second ASIC 62; however, other values may be utilized for K. Thus,for the example described above, in the preferred embodiment, K would beequal to three. If the difference in decision block 134 is greater than,or equal to, the error E, the subroutine transfers to functional block122 where the fault flag is set and then exits through block 118. If thedifference in decision block 134 is less than the error E, thesubroutine transfers directly to exit block 118 to return to the mainalgorithm.

[0051] While the preferred embodiments of the technique for detectingthe difference between an overpressure condition and sensor elementfailure has been illustrated and described for the configuration shownin FIG. 4, it will be appreciated that the technique also can beutilized with other configurations. Accordingly, the technique also canbe applied to the pressure sensor configurations shown in FIGS. 6 and 7and other dual output pressure sensor configurations not shown in theapplication. Additionally, while ASIC's have been shown for processingthe output signals from the sensor elements, it will be appreciated thatthe invention also allows the use of less expensive commerciallyavailable signal processing electronic components that are notspecifically designed for the circuit Furthermore, while the preferredembodiment of the invention has been illustrated and described for apositive gain pressure sensor, it will be appreciated that the inventionalso may be practiced with negative gain devices that have an outputthat is inversely proportional to the pressure. Such devices reachground potential for maximum pressure.

[0052] Another alternate embodiment of the invention is illustrated inFIG. 15 where blocks that are similar to blocks shown in the precedingfigures have the same numerical designators. As before, pressures P₁′and P₂′ are measured in functional block, however, the pressuresrepresent the pressure signals that have been processed by themicroprocessor in the ECU. The values P₁′ and P₂′ include the tolerancesof the sensors as follows:

P ₁ ′=P ₁ ±P _(tolerance1); and

P ₂ ′=P ₂ ±P _(tolerence2).

[0053] Note that there is no fractional gain involved in thisembodiment. Also, the threshold levels indicated in FIG. 15 as T₁′ andT₂′ correspond to the processed pressure values. The subroutine thencontinues through the absolute pressure checks described above tofunctional block 140 where an absolute value of error, E, is calculatedas:

E=|P ₁ ′−P ₂′|.

[0054] The subroutine then transfers to decision block 142 where theerror E is compared a maximum allowable error, E_(m). The maximumallowable error E_(m) is a function of the sensor tolerances and circuitcomponent errors, to include and analog to digital conversion error. Inthe preferred embodiment, the maximum allowable error is given by thefollowing formula:

E _(m)=[(P _(tolerance1) +P _(tolerance2))/P _(max)]*100 plus percentagecircuit error.

[0055] In the preferred embodiment, four percent is used for thepercentage circuit error; however, other figures also can be used.

[0056] If the error E is less than the maximum allowable error, E_(m),the subroutine returns to the main algorithm through the exit block 118.If the error E is equal to, or greater than, the maximum allowableerror, E_(m), the subroutine transfers to functional block 122 where theerror flag is set and then exits through block 118 to the mainalgorithm.

[0057] While the preferred embodiment of the invention has beenillustrated and described with a pressure sensor that included ASIC's,it will be appreciated that the invention also can be practiced with theASIC's mounted externally from the pressure sensor. For example, theASIC's could be included in the ECU. Similarly, while the preferredembodiment has been illustrated and described as utilizing an ECUmicroprocessor to compare pressure sensor data, it will be appreciatedthat other electrical components can be utilized to compare the signals.For example, the invention also can be practiced with an activeelectronic device, such as, for example, a comparator circuit,substituted for the microprocessor. Furthermore, the active electronicdevice or the microprocessor also could be included within the pressuresensor housing in lieu of being included in the ECU. Additionally, whilethe preferred embodiment of the invention has been illustrated anddescribed as being included in an ABS, it will be appreciated that theinvention also can be practice with Traction Control (TC) and/or VehicleStability Control (VSC) systems. It will also be appreciated that theflow charts shown are exemplary and that the invention can be practicedwith different combinations of the illustrated tests. Additionally, someof the specific tests can be omitted. For example, the invention can bepracticed without comparing the pressures P₁ and P₂ to the second faultthreshold T₂.

[0058] In accordance with the provisions of the patent statutes, theprinciple and mode of operation of this invention have been explainedand illustrated in its preferred embodiment. However, it must beunderstood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope.

What is claimed is:
 1. A pressure sensor assembly for a hydraulic systemcomprising: a pressure sensor housing adapted to be mounted upon ahydraulic system; a pressure sensor diaphragm carried by said pressuresensor housing; a first pressure sensing element mounted upon saidpressure sensor diaphragm; a second pressure sensing element mountedupon said pressure sensor diaphragm; a first signal conditioning circuitconnected to said first pressure sensing elements, said first signalconditioning circuit operable to generate a first pressure signal at afirst output port; a second signal conditioning circuit connected tosaid second pressure sensing elements, said second signal conditioningcircuit operable to generate a second pressure signal at a second outputport; and an electronic device connected to said output ports of saidfirst and second signal conditioning circuits, said electronic deviceoperative to compare said first and second pressure signals and, upondetecting a difference therebetween, to generate an error signal.
 2. Thepressure sensor assembly according to claim 1, wherein said electronicdevice subtracts said second pressure signal from said first pressuresignal and generates said error signal if the resulting difference isnon-zero.
 3. The pressure sensor assembly according to claim 1, whereinsaid electronic device subtracts said second pressure signal from saidfirst pressure signal and generates said error signal if the resultingdifference is greater than a predetermined threshold.
 4. The pressuresensor assembly according to claim 3 wherein said electronic deviceincludes a comparator circuit.
 5. The pressure sensor assembly accordingto claim 3 wherein said electronic device includes a microprocessor. 6.The pressure sensor assembly according to claim 5 wherein the assemblyis included in a hydraulic brake assist system and said microprocessoris further operable to disable said hydraulic brake assist system whensaid resulting difference in pressure signals is greater than saidpredetermined threshold.
 7. The pressure sensor assembly according toclaim 1 wherein said first and second signal conditioning circuits areincluded in a single electronic component.
 8. The pressure sensorassembly according to claim 7 wherein said electronic component includesan application specific integrated circuit.
 9. A pressure sensorassembly for a hydraulic system comprising: a pressure sensor housingadapted to be mounted upon a hydraulic system; a pressure sensordiaphragm carried by said pressure sensor housing; two pressure sensingelements mounted upon said pressure sensor diaphragm; a single signalconditioning circuit connected to both of said pressure sensingelements, said signal conditioning circuit operable to generate adigital pressure signal which includes pressure data from both of saidpressure sensing elements; and an electronic device connected to anoutput port of said signal conditioning circuit, said microprocessoroperative to compare said pressure data and, upon detecting a differencetherebetween, to generate an error signal.
 10. The pressure sensorassembly according to claim 9, wherein said digital pressure signal istime multiplexed.
 11. The pressure sensor assembly according to claim10, wherein said electronic device subtracts said second pressure signaldata from said first pressure-signal data and generates said errorsignal if the resulting difference is non-zero.
 12. The pressure sensorassembly according to claim 10, wherein said electronic device subtractssaid second pressure signal data from said first pressure signal dataand generates said error signal if the resulting difference is greaterthan a predetermined threshold.
 13. The pressure sensor assemblyaccording to claim 12 wherein said s electronic device includes acomparator circuit.
 14. The pressure sensor assembly according to claim12 wherein said electronic device includes a microprocessor.
 15. Thepressure sensor assembly according to claim 14 further including atemperature sensor and wherein said digital signal generated by saidsignal conditioning circuit includes temperature data.
 16. The pressuresensor assembly according to claim 14 wherein the assembly is includedin a hydraulic brake assist system and said microprocessor is furtheroperable to disable said hydraulic brake assist system when saidresulting difference in pressure signal data is greater than saidpredetermined threshold.
 17. A pressure sensor assembly for a hydraulicsystem comprising: a pressure sensor housing adapted to be mounted upona hydraulic system; a pressure sensor diaphragm carried by said pressuresensor housing; a single pressure sensing element mounted upon saidpressure sensor diaphragm; a signal conditioning circuit connected tosaid pressure sensing element, said signal conditioning circuitoperative to generate a pressure signal, said signal conditioningcircuit also including at least one diagnostic test and operable togenerate an error signal upon detecting a predetermined fault condition;and an electronic device connected to output port of said signalconditioning circuit.
 18. The pressure sensor assembly according toclaim 17 wherein said electronic device includes a comparator circuit.19. The pressure sensor assembly according to claim 17 wherein saidelectronic device includes a microprocessor.
 20. The pressure sensorassembly according to claim 19 wherein said microprocessor also includesat least one diagnostic test and is operative to generate an errorsignal upon detection of a predetermined fault condition.
 21. Thepressure sensor assembly according to claim 20 wherein the assembly isincluded in a hydraulic brake assist system and said microprocessor isoperable upon detection of an error signal to disable said hydraulicbrake assist system.
 22. The pressure sensor assembly according to claim19 wherein said signal conditioning circuit includes a plurality ofdiagnostic tests and is operative to sequentially apply said diagnostictests to detect a predetermined pressure fault condition.
 23. Thepressure sensor assembly according to claim 22 wherein saidmicroprocessor includes a plurality of diagnostic tests and is operativeto sequentially apply said diagnostic tests to detect a predeterminedpressure sensor fault condition.
 24. The pressure sensor assemblyaccording to claim 23 wherein said microprocessor is adapted to receiveoperating data from at least one vehicle component, and further whereinsaid microprocessor is operative to include said vehicle parameter datain said diagnostic test.
 25. A method for measuring a pressurecomprising: (a) providing a pressure sensor having first and secondsensor elements, the first and second sensor elements connected tocorresponding first and second signal conditioning circuits, the secondsignal conditioning circuit having a gain that is a fraction of the gainof the first signal conditioning circuit; (b) comparing the output ofthe first signal conditioning circuit to a first threshold; (c) upon theoutput of the first signal conditioning circuit exceeding the firstthreshold, comparing the output of the second signal conditioningcircuit to a second threshold; and (d) upon the output of the secondsignal conditioning circuit exceeding the second threshold, setting anerror flag.
 26. The method according to claim 25 wherein the firstthreshold is a saturation voltage for the first signal conditioningcircuit.
 27. The method according to claim 26 wherein the secondthreshold is equal to the first threshold.
 28. The method according toclaim 26 wherein the second threshold is less than the first threshold.29. A method for measuring a pressure comprising: (a) providing apressure sensor having first and second sensor elements, the first andsecond sensor elements connected to corresponding first and secondsignal conditioning circuits, the second signal conditioning circuithaving a gain that is a fraction of the gain of the first signalconditioning circuit; (b) comparing the output of the first signalconditioning circuit to a first threshold and, upon the output of thefirst signal conditioning circuit exceeding the first threshold, settingan error flag; (c) upon the output of the first signal conditioningcircuit being less than or equal to the first threshold, comparing theoutput of the second signal conditioning circuit to the first thresholdand upon the output of the second signal conditioning circuit exceedingthe first threshold, setting an error flag; (d) upon the output of thesecond signal conditioning circuit being less than or equal to thethreshold, comparing the output of the first signal conditioning circuitto a second, and (e) upon the output the first signal conditioningcircuit being less than the second threshold, comparing the output ofthe second signal conditioning circuit to a third threshold and, uponthe output of the second signal conditioning circuit being less than thethird threshold, setting an error flag.
 30. The method according toclaim 29 further including, subsequent to step (e), the following steps:(f) upon the output of the second signal conditioning circuit beingbetween the first and third thresholds, determining the absolute valueof the difference between the output of the first signal conditioningcircuit and the product of a constant times the output of the secondsignal conditioning circuit; (g) comparing the difference between theoutputs to an error value; and (h upon the difference being greater thanthe error value, setting an error flag.
 31. The method according toclaim 30 wherein the multiplier constant for the output of the secondsignal conditioning circuit is a function of the gain of the secondsignal conditioning circuit.
 32. The method according to claim 31wherein the multiplier constant for the output of the second signalconditioning circuit is the reciprocal of the fractional reduction ofthe gain of the second signal conditioning circuit relative to the gainof the first signal conditioning circuit.
 33. The method according toclaim 25 wherein, prior to step(b), the output of the first signalconditioning circuit is compared to a third threshold that is less thatthe first and second thresholds and, upon the output of the first signalconditioning circuit being less than the third threshold, setting anerror flag.
 34. A method for measuring a pressure comprising: (a)providing a pressure sensor having first and second sensor elements; (b)determining the absolute value of the difference of the outputs of thefirst and second sensor elements; (c) comparing the absolute value ofthe difference of the outputs of the first and second sensor elements toa maximum difference factor; and (d) upon the absolute value of thedifference of the outputs of the first and second sensor elementsgreater than the maximum difference factor, setting an error flag. 35.The method according to claim 34 wherein the outputs of the first andsecond pressure sensor elements include pressure sensor tolerances. 36.The method according to claim 35 wherein the maximum difference factoris a function of the pressure sensor tolerances and the circuitcomponent errors.
 37. The method according to claim 29 furtherincluding, subsequent to step (e), the following steps: (f) determiningthe absolute value of the difference of the outputs of the first andsecond sensor elements; (g) comparing the absolute value of thedifference of the outputs of the first and second sensor elements to amaximum difference factor; and (h) upon the absolute value of thedifference of the outputs of the first and second sensor elementsgreater than the maximum difference factor, setting an error flag. 38.The method according to claim 37 wherein the outputs of the first andsecond pressure sensor elements include pressure sensor tolerances. 39.The method according to claim 38 wherein the maximum difference factoris a function of the pressure sensor tolerances and the circuitcomponent errors.